Side-scanning sonars (SSS) are designed to create visual images of the sea floor and any objects lying thereon. The image is created by scanning the area of interest with acoustic beams. A short acoustic pulse, typically on the order of a millisecond, is launched from the projector array, and it travels away from the array at the sound propagation velocity of the medium, typically about 1500 m/s in sea water. The sea floor and any objects in the sound field reflect a portion of the sound back to the hydrophone array where it is sensed. The two-way travel time of the echos provides a very accurate measure of the distance to the reflecting objects in a direction normal to the major axis of the acoustic array called the cross-track direction. Scanning in the along-track direction is accomplished by physically moving the array while periodically emitting new acoustic pulses. The result is a two-dimensional map of the reflecting objects in the scanned area. The echo amplitudes recorded are related to the size, shape, orientation, and acoustic impedance of the reflecting objects at each spatial location.
The shape of the transmitted acoustic field, i.e., the projector beam pattern, as well as the shape of the receiver array pattern (and more particularly, the shape of their intersection), must be controlled very carefully to produce accurate images. There are a significant number of different approaches to their design, but all have quite broad beams in the vertical plane, typically around 90 degrees centered at 45 degrees below the horizon, and quite narrow beams in the horizontal plane (because the latter defines the sonar resolution in the along-track direction). One or more beams 11 containing the transmitted pulses 12 are produced on each side of the towfish vehicle 13 as illustrated in FIG. 1.
In fact, most modern side-scanning sonars produce extremely narrow search beams on the order of 0.1 degrees in the along-track direction, and this creates significant difficulties in the measurement and calibration process. The hydrophone array itself is usually operated in the near field, or Fresnel zone where range &lt;&lt;L.sup.2 /.lambda., where L is the total array length and .lambda. is the acoustic wavelength, which means it must be focused to produce useful beams. The depth of field is usually so short that the focal distance must be changed for different target distances. This is usually accomplished by subdividing the receiving array into a number of discrete transducer elements and electronically introducing a time delay or phase shift into the received signal that focuses the beam dynamically as the pulse travels. Additionally, side-scan sonars, unlike sector-scan sonars, must change their beam width as the pulse travels so as to maintain a constant along-track resolution at all ranges. This is usually accomplished by electronically changing the length of the hydrophone according to EQU N.sub.c =Sr.lambda./d.DELTA.x,
where N.sub.c is the number of array element signals summed to form the beam, Sr is the slant range to the target, d is the element spacing, and .DELTA.x is the desired along-track resolution. This also requires that the array be divided into a number of discrete elements.
It should be noted that the definition of resolution differs greatly between users in various specialties, sometimes resulting in confusion. Designers of echo ranging sensors, such as sonars and radars, define resolution in the cross-track direction as the half-power points on the autocorrelation function of the echo-ranging signal. For the special but commonly encountered case of a gated single-frequency sinusoid, the cross-track slant range resolution of the sensor is given by EQU .DELTA.Sr=cT/2,
where c is the propagation velocity of the energy in the medium and T is the time duration of the transmitted signal. This measure of resolution reveals the capability of the sonar to resolve two point reflectors in close proximity to each other. By contrast, image processing specialists define image resolution as the spatial sampling intervals represented by the image data set. Designers of image display devices, on the other hand, define resolving power as the number of line pairs per unit length that can just be resolved by the output device, where line pairs refers to alternating black and white lines whose width is equal to their spacing. The space occupied by a line pair is equal to the spatial period of the pattern, or the inverse of its square wave frequency. The latter is measured in line pairs per unit length.
Although the dimension of interest in sea bottom profiling is horizontal distance in the cross-track direction, what is actually measured with considerable accuracy is the slant range from the sonar. The conversion from slant range to horizontal range is accomplished by use of the Pythagorean theorem and the assumption that the bottom is horizontal and flat in a gross sense. That is, horizontal range is given by EQU Hr=(Sr.sup.2 -He.sup.2)1/2,
where Sr is slant range and He is the height of the sonar above the bottom, as illustrated in FIG. 2. The resolution of the sonar in the horizontal cross-track direction is affected by the grazing angle (.lambda.), and can be calculated from EQU .DELTA.Hr=.DELTA.Sr/cos .lambda..
Prior Art. The receive array beam pattern is the spatial equivalent of the system impulse response: it is necessary and sufficient to completely specify the system transfer function and thus to allow performance predictions for any operating environment of interest. The usual method is to obtain a measurement of the pattern in a quasi-static situation, i.e., in a test pool under carefully controlled conditions with the array moving very slowly so that a large number of samples can be collected to provide a smooth estimate of the beam pattern. Patterns are usually obtained by rotating the array of interest while receiving pulses from a calibrated projector at a fairly short distance and recording the amplitude versus bearing angle. Alternately, especially for side-scanning sonars, i.e., those producing constant along-track resolution rather than constant angular resolution, a translational pattern may be obtained.
For high resolution side-scanning sonar patterns at long ranges, this process requires a considerable amount of time. For example, measurements of the one-way receive pattern of a 0.1 degree sonar array at a range of 100 m, to obtain a sampling density of at least 10 sample points in the main lobe to provide a smooth pattern, requires spatial sampling intervals of 1.7 cm and a maximum array velocity of 13 cm/s.
Unfortunately, there are several problems with this approach. First, the static patterns do not take into account the effects of motion, such as flow noise, towfish vibration, inaccurate forward velocity estimates, yawing, and side slip. For long arrays, hydrostatic pressure loading on certain types of array construction is not accounted for in a shallow test pool. The effects of temporal and spatial nonhomogeneity in the open sea are not accounted for. Arrays are usually removed from their towfish when being tested in a test tank: several potentially degrading effects are thereby unaccounted for, such as acoustic reflections from the towfish and array deformation due to being rigidly secured to the towfish. Further, it is obvious that normal beam pattern cannot be measured at sea because sonar operating speeds are typically on the order of 50 times that required for good pattern definition.
Finally, and perhaps most importantly, a beam pattern doesn't really provide a useful qualitative measure of image quality to be expected from the sonar, nor does it provide any information at all about the resolution achieved in the range direction. What is required is a means of measuring resolution performance actually being achieved by the sonar while operating in the field under normal operating conditions, and in a way that directly relates to image quality.
In the past, all of this has been largely ignored because no method was available to solve the problem. Efforts to measure SSS resolution and image quality have been very expensive, non quantitative, not repeatable, and not useful for new design because experiment parameters could not be quantified or controlled. Typically, a field of objects of interest is laid according to some plan. The area is almost never surveyed because of the time and cost involved, so the actual location and orientation of targets is usually unknown; and little or nothing is known about the existence, location, and identification of other objects, either natural or man-made. Because of the time required to establish the field and run trials, and the fact that tests are conducted where people, fish, and other sea life are present unbeknownst to the users of the field, targets are frequently disturbed. Also, the size of targets used are often close to that of the sonar resolution, so the limits of resolution are not readily discernible from the resulting images. It will be well known to those skilled in the art that results using this method are very dependent on the type of bottom at the test site, and are thurs almost impossible to reproduce at another test site.